Not Applicable
The present invention relates to a process for producing a thin-film solar or photovoltaic module as well as to separating means for use in this process.
One possibility for producing thin-film solar modules or thin-film photovoltaic modules comprising a plurality of solar or photovoltaic cells disposed in parallel on a common substrate, which are produced and electrically interconnected by a plurality of cell-overlapping coating steps and coating-separation steps during cell production has already been described in U.S. Pat. No. 4,243,432. In that specification, CdS-CuxS solar cells are produced in that a glass substrate is always completely provided with deposited individual layers, one upon the other. The layers in form of thin films mainly comprise a hard contact layer of SnOx serving as the lower electrode and a pn double layer of CdS-CuxS disposed on it as well as a further contact layer serving as the upper electrode. Cuts performed between the individual thin film deposition steps have been provided to subdivide the cell-overlapping substrate film coating into individual cells, on the one hand, and to electrically series-connect the cells of a substrate, on the other hand. For serial interconnection of the cells, the cuts are provided such that the upper electrode of a cell 1 contacts the lower electrode of following cell 2, however, is separated from the upper electrode of cell 2. This scheme is continuously repeated over the whole substrate length. Various methods for separating the individual layers have been suggested in this US patent, inter alia by ultrasonic techniques, and it has been stated that the pn layer may also be separated for instance by means of a rotating or a non-rotating cutting tool.
In U.S. Pat. No. 4,315,096, a similar process has been disclosed for CdTe and CdS layers wherein the separation cuts having a width of between 5 xcexcm and 100 xcexcm are also produced either by mechanical methods not defined in detail or by means of laser beams. U.S. Pat. No. 5,501,744, also, refers to the production of CdTe and CdS solar cells. According to U.S. Pat. No. 5,501,744, the modules are processed by means of a tool head which is movable both in transversal and longitudinal direction relative to the substrate. The tool head includes lasers, arranged side by side, sandblast blowers or deposition means which are obliquely directed to the substrate and, preferably, do not contact the substrate. After processing the substrate in longitudinal direction, the head is displaced in transverse direction to a new starting position for longitudinal movement. During the course of this transverse movement, the processing units are not activated.
As yet, however, commercially traded silicon solar cell modules have not, as a rule, been fabricated in accordance with the above process of U.S. Pat. No. 4,243,432. Rather, they have been made of individual cells which are interconnected by means of soldered-on metal strips. Modules comprising CdS-CuxS solar cells have not so far been produced on an industrial scale in accordance with the above-referenced process of U.S. Pat. No. 4,243,432. The same applies to CdS/CdTe solar cells which have, for a considerable time been considered as promising and which include a front contact of a transparent TCO layer, mostly in the shape of a so-called ITO layer. As to the production and the structure of such an individual solar cell, explicit reference is made to European Patent No. EP 0,535,522, which corresponds to U.S. Pat. No. 5,304,499.
The present invention provides a process for producing a thin film solar module which is characterized in that a scraping tool as cutting or separating means for the pn layer is used. Contrary to common cutting means, the cutting tool employed in accordance with the present invention includes a flattened tip and the plane flattened surface serves as sliding surface of the tool. During the separating process, the tool is guided so that it slides with its plane sliding surface on the first contact layer while the sliding surface rests with its complete flattened face on this layer which is in parallel to it. The longitudinal axis of the tool is in this case perpendicular to the contact layer, or the substrate, respectively. By this design and arrangement, the danger of damage to the contact layer by the plane sliding surface of the tool is reduced to a minimum. The adjustment of the pressing force exerted on the tool is substantially uncritical. The force may, without any danger, be adjusted at such a high value that the pn double layer is safely cut through. It is not necessary to provide for any sophisticated regulations and adjustments for the scribing or scraping depth of the tool. In addition, the tool may without any complicated adjustment be moved in any direction and may for instance follow a meander-shaped path without any need of being axially rotated before being forwarded to a new work area and before being moved in a new processing or cutting direction.
Contrary to the normally common technique of the application of non-rotating cutting tools such as raising and lowering tools, the cutting tool of the process according to the invention need not to be raised from the coated substrate, neither during the cutting process nor when shifting the tool to a new work area. By avoiding a time consuming raising and lowering of the tool on and from the front contact layer, respectively, it was possible to increase the life time of the cutting tool and to enhance the efficiency of the process.
It was an object of the present inventors to provide a process by which an efficient production of thin film solar modules is possible while making use of cell-overlapping thin film deposition and a rapid and safe separating technique. A further object was to develop suitable separating means for the production of thin film solar modules, which make such rapid and, at the same time, safe separation possible. Rapid separation is necessary because the separation steps are part of the continuous production process and cannot be decoupled from it. During the course of the separation process, in addition, it should be ensured that the desired layers are sufficiently completely cut through and separated while the layers or thin films disposed under them remain undamaged.
The prior art scribing tools which are drawn, in an inclined position and without a parallel sliding surface, over a substrate do not show the advantages described above. U.S. Pat. No. 4,502,225 uses a scribing tool which is moved under an oblique angle of preferably 75xc2x0 relative to the horizontal (so-called back racking angle) over a substrate in order to separate silicon layers. The tool preferably includes a rounded diamond tip. As alternatives for the obliquely disposed tool, pyramid-like or truncated tips have been referred to. According to this US patent, the pressure adjustment of the tool and of its sharp scribing tip having a preferred diameter of about 0.01 mm is very critical. In order to obtain a defined scribing depth of the inclined tool rounded-off at the tip into the layer to be cut while not damaging the underlying layer, the pressing force is adjusted to tight limits using a complicated mechanical facility. One tries to monitor successful layer separation by means of resistance measurements, an action which can be ruled out in the case of electrically not conducting layers. After cutting a layer segment over the whole substrate length, the tool is raised and shifted into the new scribing position where it is lowered again. The oblique scribing tool shown can be moved in one direction only.
U.S. Pat. No. 4,589,194 shows also a scribing stylus guided in an inclined position relative to the substrate surface which as an alternative to the preferred diamond tip may optionally be produced completely of a hard metal. Amorphous silicon layers and back contact layers disposed thereon are cut. In order to avoid that the TCO layer disposed underneath be damaged by the sharp tool tip of only 0.006 mm diameter maximum as well as by the relatively high pressing forces as revealed, the Si layer is not completely cut but rather only so far that a certain thickness of the Si layer remains on the TCO layer. It is for this reason that the stylus has to be adjusted very sensitively. The stylus tip is ultrasonically driven. For shifting to a new scribing position, the inclined stylus is raised from the substrate.
As compared to these scribing tools which have to be drawn in an inclined position over the coated substrates, the adjusting measures on the scraping tool of the present invention with its sliding surface parallel to the substrate are considerably less complicated. In addition, the scraping tool may without being raised, be moved at a high speed and in a precise and stable movement into various directions. The tool according to the invention is guided perpendicularly over the thin film carrying substrate and slides with its sliding surface perpendicular relative to its longitudinal axis safely and gently on the harder contact layer and separate the overlaying layer reliably and completely.
The process of the invention is particularly suited for such CdS/CdTe solar cells as referred to having a front contact of a hard and smooth TCO layer such as the ITO layer which is about ten times harder than the soft, brittle pn layer. Thereby, it is possible to cut the pn layer while not penetrating into the front contact material on which the cutting tool slides with its flattened tip when cutting the pn layer. The process is particularly efficient since the cutting tool need not be raised before moving it on to a new starting position. It is only shifted in a second transverse moving direction to reach a new starting position before it is moved in a now-changed, generally opposite, longitudinal processing direction over the substrate. In this manner, the tool follows a meander-shaped path: It is moved in a first longitudinal direction over the whole module length, shifted in a transverse second moving direction in a marginal region of the module and moved in a longitudinal direction opposite to said first longitudinal direction. With a view to an efficient procedure, this present advantage cannot not be obtained by using cutting tools having a rotating cutting head or cutting tools periodically raised from, and lowered to, the substrate. This also applies to the above-described scribing tools that have to be guided in an inclined position.
Users who do not wish to shift the tools within a marginal area of the modules in said second transverse moving direction without raising the tool before it reaches a new starting position, may of course accept the prolongation of the process caused by the raising and lowering of the tool. When arranging a sufficient number of tools in parallel side-by-side in order to process the complete module in one working step, to raising and lowering during module coating does not apply anyway.
Since preferably the whole substrate is provided with the pn layer, the cutting tool when being shifted in said second moving direction cuts the pn layer and slides on the front contact layer within a marginal region of the substrate. The process also covers a case in which the cutting tool is shifted in the second moving direction in an edge area of the substrate, which is not provided with a pn layer on the front contact layer.
When shifting on in the second transverse moving direction, it is not the first cutting edge of the cutting tools which is used but rather a different tool side which is moved facing the second moving direction. After having been shifted to a new starting position, the cutting tool which is preferably provided with a rectangular sliding surface is moved, preferably again without being raised and without being rotated about its longitudinal axis, opposite to the first moving direction.
It is of a constructional advantage to perform the relative movement in the first moving direction during which the pn layer is cut by only moving the cutting tool, or the cutting tools, respectively, over the substrate.
Preferably, the cutting tool is resiliently pressed with its sliding surface against the thin film carrying substrate. Sophisticated adjustment measures of fixedly chucked cutting tools as required in conventional workpiece processing techniques are not necessary. By means of the cutting tool of the invention resiliently pressed against the front contact layer, the pn layer is always reliably cut.
By further optimizing the operation of the cutting tool, it was possible to make the process still more efficient since at the beginning of the cutting tool operation, a tool is used the tip flattening of which has a considerable small value which in view of abrasion increases during the operation period or tool life. In the process, the operation period is limited so that flattening and increase of the plane sliding surface remains within a tolerable range in which the tool still continues to cut sufficiently. As compared thereto, the initial tip flattening is rather small at the beginning, however, already so large that the tool tip slides on the first contact layer without damaging it. In this connection, the pressure force of the tool depending on the adjusted spring force plays a role as well. Preferably, the pressure force is increased with the abrasion-caused increase of the area of the flattened tip in order to constantly produce a constant uniform optimum pressure. The force exerted by the spring is adjusted so that, on one hand, the first contact layer is not damaged and, on the other hand, sliding-up of the tool is avoided. In the production plant, the pressure force of the tools of the invention was adjusted only twice within a week while the tools were in uninterrupted operation.
The cutting means of the invention constitutes a cutting tool which is simple and economical to produce. Based on these properties, the exchangeable tool is suitable for the aimed-at mass production of thin film solar modules.
It has been found that particularly large separation distances of up to 20 km could be obtained by using a rectangular sliding surface and preferably unequally long cutting edges without having to exchange the tool and to interrupt the production process. Furthermore, it is possible to work with a tip of the kind of an inverted four-sided truncated pyramid as tool tip by which, under equal cutting conditions, the first moving direction, or main scraping direction, is reversible.
It has also been found that by using the unequally long cutting edges, in the main scraping direction and in the moving or shifting direction preferably rectangular thereto an optimum relation of the force to be applied onto the cutting tool and the surface pressure can be obtained. The cutting tool according to the invention has the additional advantage that even in case of a slightly oblique or angular run satisfactory cuts are made while, in addition to the cutting edge in the main scraping direction, the cutting edge in the moving or shifting direction comes into operation. Thereby it is possible to provide for parallel cuts on the substrate without the need of complicated readjustments of the relative angular orientations of substrate and tool.
Adjustment, mounting and movement of the tool perpendicular on the substrate can be obtained with little mechanical endeavors.
In the case of a CdS/CdTe pn double layer wherein the CdS layer has a thickness in the range of only 100 nm or less, the thin CdS layer adheres so strongly to the first contact layer that it is not removed by the cutting tool. Since, however, the CdS layer is so thin, it practically shows the hardness of the hard first contact layer underneath. For the flattened tool tip, therefore, identical sliding conditions are obtained. In case of thicker CdS layers, their brittleness becomes more and more evident so that they are removed together with the CdTe layer.